Use of a liquid aluminum chloride complex catalyst in production of sec-butylbenzene from benzene and n-butene has heretofore been known. For example, JP-A-50-137933 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method in which a liquid aluminum chloride complex catalyst is used in such an amount that the amount of aluminum chloride is from 0.05 to 0.25% by weight of the reaction mixture.
In production of sec-butylbenzene from benzene and n-butene by an alkylation method, the product is a mixture mainly containing sec-butylbenzene (SBB), isobutylbenzene (IBB), disec-butylbenzene (DSBB), and tri-sec-butylbenzene (TSBB).
Of these compounds, di-sec-butylbenzene and tri-secbutylbenzene are each separated from the reaction mixture and then transalkylated into sec-butylbenzene. This reaction can be illustrated as follows: ##STR1##
Boiling points of isobutylbenzene and sec-butylbenzene are 172.8.degree. C. and 173.5.degree. C., respectively, and are close to each other. Thus it is difficult to separate these two compounds from each other by distillation. Isobutylbenzene formed as a by-product in the reaction is sent as such to an air oxidation step along with sec-butylbenzene. It is known, however, that if sec-butylbenzene contains isobutylbenzene, the rate of reaction in the air oxidation step is markedly decreased (see JP-A-48-80524). For example, the rate of air oxidation of sec-butylbenzene, when the sec-butylbenzene contains 1% by weight of isobutylbenzene, decreases to about 91% of that when the sec-butylbenzene does not contain isobutylbenzene at all. Similarly, when the isobutylbenzene content is 1.65% by weight, the rate of air oxidation decreases to about 86%; when the isobutylbenzene content is 2% by weight, the rate of air oxidation decreases to about 84%; and when the isobutylbenzene content is 3.5% by weight, the rate of air oxidation decreases to as much as about 82%. Therefore, in order to efficiently undergo the air oxidation step, it is necessary to use secbutylbenzene having a decreased isobutylbenzene content as much as possible. For this reason, the amount of isobutylbenzene formed as a by-product at the step of production of secbutylbenzene from benzene and n-butene is needed to minimize.
However, when conventional alkylation methods are followed, maintenance of a satisfactory level of the yield of the desired sec-butylbenzene is attended by an increased proportion of isobutylbenzene formed as a by-product, reaching 1 to 4% by weight based on the desired sec-butylbenzene, and such a large amount of isobutylbenzene formed as a by-product has been subjected to air oxidation without being removed.